Aliphatic polyesters, such as polyglycolic acid and polylactic acid, can be decomposed by microorganisms or enzymes present in nature, such as soil or sea water, so that they are noted as biodegradable polymer materials giving little load to the environment. Further, aliphatic polyesters are utilized as polymer materials for medical use, such as sutures for surgery or artificial skin, since they can be decomposed or absorbed in vivo.
Among the aliphatic polyesters, polyglycolic acid is excellent in gas-barrier properties, such as oxygen gas-barrier property, carbon dioxide gas-barrier property and water vapor-barrier property and also excellent in heat resistance and mechanical properties, and therefore the development of new use thereof is under way singly or in a composite state together with another resin material in the fields of packaging materials, etc.
An aliphatic polyester can be synthesized by dehydro-polycondensation of an α-hydroxycarboxylic acid, such as glycolic acid or lactic acid, but in order to effectively synthesize an aliphatic polyester of a high molecular weight, there has been generally adopted a process of synthesizing a bimolecular cyclic ester of an α-hydroxycarboxylic acid and subjecting the cyclic ester to ring-opening polymerization. For example, by ring-opening polymerization of glycolide that is a bimolecular cyclic ester or cyclic diester of glycolic acid, polyglycolic acid is obtained. By ring-opening polymerization of lactide that is a bimolecular cyclic ester of lactic acid, polylactic acid is obtained.
The polymerization process for producing an aliphatic polyester by ring-opening polymerization of a cyclic ester is generally performed in the form of melt-polymerization where all the steps are operated above the melting point of the product aliphatic polyester, and the present inventors, et. al., have proposed a process wherein the latter half step of the ring-opening polymerization of cyclic ester is performed in a reactor of relatively thin tubes to effect solid-phase polymerization (Patent Document 1 listed below). By adopting the solid-phase polymerization, it becomes possible to attain an advantage that the product polyester is easily recovered in the form of lumps while causing a volumetric shrinkage and release thereof to be released from the inner surfaces of the tubes.
However, in the ring-opening polymerization of a cyclic ester including the above-mentioned case of solid-phase polymerization, it is inevitable that some amount (referred to as 2-8% in Patent Documents 2-4 shown below) of non-reacted cyclic ester monomer remains in the product aliphatic polyester. The residual monomer causes a lowering in melt-extrudability and stretchability of the product aliphatic polyester and also degradation or fluctuation of properties of the product obtained therefrom (e.g., cutting of filaments or local lowering of properties of films), so that it is desirable to reduce the residual monomer as small an amount as possible. For this reason, several methods have been proposed in order to reduce the residual monomer in the product polymers (e.g., Patent Documents 2-4 shown below). For example, Patent Document 2 discloses a process wherein a high-temperature dry gas is caused to contact particles of product polyester (milled product), thereby providing polyglycolic acid with a reduced residual monomer down to possibly ca. 0.2%. Further, Patent Document 3 describes that the process of Patent Document 2 is inefficient because the process requires a treatment time of several tens of hours in order to achieve a reduction down to 2% or less, and proposes a reduced pressure treatment of a resultant polymer in a molten state. Patent Document 4 also proposes a reduced pressure treatment of a resultant polymer in a molten state.    Patent Document 1: WO-A 03/006526    Patent Document 2: U.S. Pat. No. 3,565,869    Patent Document 3: JP-A 3-14829    Patent Document 4: JP-A 9-12690
However, according to any of the processes of the above-mentioned Patent Documents, the reduction of residual monomer was insufficient and it was difficult to obtain an aliphatic polyester with a residual monomer of 0.3 wt. % or less, so adverse effects thereof to the lowering in properties of the aliphatic polyesters remained to be a non-ignorable level.